This invention relates to a stabilizer system for polyesters and their commercially useful stabilized compositions which provide finished products having improved weatherability.
Polyesters of various compositions have been used extensively to prepare film, sheets and various molded, shaped or formed articles. In order for these various articles to be successfully used in outdoor applications they must retain their appearance, particularly with respect to gloss, transparency and shape and a reasonable amount of physical properties, i.e., impact strength, tensile properties, etc. Many of such articles, however, tend to degrade and the physical properties of the articles are reduced when exposed to outdoor conditions (e.g., ultraviolet light, heat, water and oxygen) over an extended period of time, thus rendering such articles less useful for most outdoor applications. Therefore, considerable effort has been directed to improving the weatherability of such polymeric compositions.
Generally the existing stabilizers and stabilizer combinations have not been effective in preventing surface degradation of polyester compositions for periods of time sufficient for outdoor applications. It is generally thought that a plastic suitable for outdoor use will have good retention of physical and appearance properties for at least 2000-3000 hours exposure in an accelerated weathering device such as an Atlas XWR Weather-Ometer.RTM.. However, obtaining homopolyesters and copolyesters suitable for outdoor use--that is, polyesters which retain both the desired physical properties and appearance properties after long term outdoor exposure--has heretofore not been possible. Typically, articles formulated from polyester compositions stabilized with known stabilizers and stabilizer combinations retain the desired appearance and/or physical properties generally for less than 1000 hours exposure in an accelerated weathering device. Upon exposure for longer periods such polyester articles rapidly lose the desired appearance properties, i.e. becoming hazy, dull and often misshapen, and/or key physical properties, i.e. impact strength.
It has unexpectedly been found that articles formulated from certain aliphatic dicarboxylic acid based homopolyesters and copolyesters, when stabilized with selected conventional ultraviolet light (UV) stabilizers in combination with certain hindered amine light stabilizers (HALS), can withstand significantly longer periods of exposure under accelerated weathering conditions with good retention of both appearance properties (gloss, transparency, shape) and physical properties (impact and tensile strength) than has previously been possible. It should be readily apparent to those skilled in the art that polyesters having dramatically increased weatherability as exhibited by the polyesters of this invention will be especially suitable for use in many outdoor applications.